Re: [Crash-utility] uniquely identifying KDUMP files that originate from QEMU

----- Original Message ----- > From: HATAYAMA Daisuke <d.hatayama(a)jp.fujitsu.com&gt; > To: ptesarik(a)suse.cz > Cc: lersek(a)redhat.com, kexec(a)lists.infradead.org > Subject: Re: uniquely identifying KDUMP files that originate from QEMU > Message-ID: > <20141112.120838.303682123986142686.d.hatayama(a)jp.fujitsu.com&gt; > Content-Type: Text/Plain; charset=us-ascii > > From: Petr Tesarik <ptesarik(a)suse.cz&gt; > Subject: Re: uniquely identifying KDUMP files that originate from QEMU > Date: Tue, 11 Nov 2014 13:09:13 +0100 > > > On Tue, 11 Nov 2014 12:22:52 +0100 > > Laszlo Ersek <lersek(a)redhat.com&gt; wrote: > > > >> (Note: I'm not subscribed to either qemu-devel or the kexec list; please > >> keep me CC'd.) > >> > >> QEMU is able to dump the guest's memory in KDUMP format (kdump-zlib, > >> kdump-lzo, kdump-snappy) with the "dump-guest-memory" QMP command. > >> > >> The resultant vmcore is usually analyzed with the "crash" utility. > >> > >> The original tool producing such files is kdump. Unlike the procedure > >> performed by QEMU, kdump runs from *within* the guest (under a kexec'd > >> kdump kernel), and has more information about the original guest kernel > >> state (which is being dumped) than QEMU. To QEMU, the guest kernel state > >> is opaque. > >> > >> For this reason, the kdump preparation logic in QEMU hardcodes a number > >> of fields in the kdump header. The direct issue is the "phys_base" > >> field. Refer to dump.c, functions create_header32(), create_header64(), > >> and "include/sysemu/dump.h", macro PHYS_BASE (with the replacement text > >> "0"). > >> > >> http://git.qemu.org/?p=qemu.git;a=blob;f=dump.c;h=9c7dad8f865af3b778589dd... > >> > >> http://git.qemu.org/?p=qemu.git;a=blob;f=include/sysemu/dump.h;h=7e4ec5c7... > >> > >> This works in most cases, because the guest Linux kernel indeed tends to > >> be loaded at guest-phys address 0. However, when the guest Linux kernel > >> is booted on top of OVMF (which has a somewhat unusual UEFI memory map), > >> then the guest Linux kernel is loaded at 16MB, thereby getting out of > >> sync with the phys_base=0 setting visible in the KDUMP header. > >> > >> This trips up the "crash" utility. > >> > >> Dave worked around the issue in "crash" for ELF format dumps -- "crash" > >> can identify QEMU as the originator of the vmcore by finding the QEMU > >> notes in the ELF vmcore. If those are present, then "crash" employs a > >> heuristic, probing for a phys_base up to 32MB, in 1MB steps. > >> > >> Alas, the QEMU notes are not present in the KDUMP-format vmcores that > >> QEMU produces (they cannot be), > > > > Why? Since KDUMP format version 4, the complete ELF notes can be stored > > in the file (see offset_note, size_note fields in the sub-header). > > > > Yes, the QEMU notes is present in kdump-compressed format. But > phys_base cannot be calculated only from qemu-side. We cannot do more > than the efforts crash utility does for workaround. So, the phys_base > value in kdump-sub header is now designed to have 0 now. > > Anyway, phys_base is kernel information. To make it available for qemu > side, there's need to prepare a mechanism for qemu to have any access > to it. > > One ad-hoc but simple way is to put phys_base value as part of > VMCOREINFO note information on kernel. > > Although there has already been a similar one in VMCOREINFO, like > > arch/x86/kernel/ > == > void arch_crash_save_vmcoreinfo(void) > { > VMCOREINFO_SYMBOL(phys_base); <---- This > VMCOREINFO_SYMBOL(init_level4_pgt); > > ... > == > > this is meangless, because this value is a virtual address assigned to > phys_base symbol. To refer to the value of phys_base itself, we need > the phys_base value we are about to get now. > > So, instead, if we change this to save the value, not value of symbol > phys_base, we can get phys_base from the VMCOREINFO. > > The VMCOREINFO consists simply of string. So it's easy to search > vmcore for it e.g. using strings and grep like this: > > $ strings vmcore-3.10.0-121.el7.x86_64 | grep -E ".*VMCOREINFO.*" -A 100 > VMCOREINFO > OSRELEASE=3.10.0-121.el7.x86_64 > PAGESIZE=4096 > ... > SYMBOL(phys_base)=ffffffff818e5010 <-- though this is address of phys_base > now... > SYMBOL(init_level4_pgt)=ffffffff818de000 > SYMBOL(node_data)=ffffffff819f1cc0 > LENGTH(node_data)=1024 > CRASHTIME=1399460394 > ... > > This should also be useful to get phys_base of 2nd kernel, which is > inherently relocated kernel from a vmcore generated using qemu dump. > > This is far from well-designed from qemu's point of view, but it would > be manually easier to get phys_base than now. > > Obviously, the VMCOREINFO is available only if CONFIG_KEXEC is > enabled. Other users cannot use this. > > -- > Thanks. > HATAYAMA, Daisuke I agree that the actual value of phys_base should be included in the vmcoreinfo. However, it won't help in this case because the vmcoreinfo data is not copied into the compressed dumpfile header. The offset_vmcoreinfo and size_vmcoreinfo fields are zero.

> This is far from well-designed from qemu's point of view, but it would > be manually easier to get phys_base than now.

Here's an example header dump of a QEMU-generated dumpfile: crash> help -n makedumpfile header: signature: "makedumpfile" type: 1 version: 1 all_flat_data: num_array: 18695 array: 7f484b760010 file_size: 0 diskdump_data: filename: vmcore.ovmf.rhel7.kdump-snappy flags: c6 (KDUMP_CMPRS_LOCAL|ERROR_EXCLUDED|LZO_SUPPORTED|SNAPPY_SUPPORTED) [FLAT] dfd: 3 ofp: 3e441b1260 machine_type: 62 (EM_X86_64) header: 1a68fe0 signature: "KDUMP " header_version: 6 utsname: sysname: nodename: release: version: machine: x86_64 domainname: timestamp: tv_sec: 0 tv_usec: 0 status: 4 (DUMP_DH_COMPRESSED_SNAPPY) block_size: 4096 sub_hdr_size: 1 bitmap_blocks: 76 max_mapnr: 1245184 total_ram_blocks: 0 device_blocks: 0 written_blocks: 0 current_cpu: 0 nr_cpus: 4 tasks[nr_cpus]: 0 0 0 0 sub_header: 0 (n/a) sub_header_kdump: 1a69ff0 phys_base: 0 dump_level: 1 (0x1) (DUMP_EXCLUDE_ZERO) split: 0 start_pfn: (unused) end_pfn: (unused) offset_vmcoreinfo: 0 (0x0) size_vmcoreinfo: 0 (0x0) offset_note: 4200 (0x1068) size_note: 3232 (0xca0) num_prstatus_notes: 4 notes_buf: 1a6b000 notes[0]: 1a6b000 notes[1]: 1a6b164 notes[2]: 1a6b2c8 notes[3]: 1a6b42c NT_PRSTATUS_offset: 1068 11cc 1330 1494 offset_eraseinfo: 0 (0x0) size_eraseinfo: 0 (0x0) start_pfn_64: (unused) end_pfn_64: (unused) max_mapnr_64: 1245184 (0x130000) data_offset: 4e000 block_size: 4096 block_shift: 12 bitmap: 7f484b713010 bitmap_len: 311296 max_mapnr: 1245184 (0x130000) dumpable_bitmap: 7f484b6c6010 byte: 0 bit: 0 compressed_page: 1a8c660 curbufptr: 1a7f650 ... Note that QEMU does add self-generated register dumps above, but the special "QEMU" note that is added to ELF kdumps is not included.

Also note that the kernel version information is also left zero-filled.

In any case, if either a QEMU note or a diskdump.data flag were added, I would be more than happy. Dave